1. Field of the Invention
This invention relates to optical spectral filters such as Bragg gratings and more specifically to a waveguide tunable Bragg grating that uses compliant MEMS technology.
2. Description of the Related Art
Bragg gratings transmit light at wavelengths in a predetermined passband and reflect the light at wavelengths that lie outside the passband back towards the source. A Bragg grating is a comparatively simple structure consisting of an arrangement of parallel semi-reflecting plates separated by a suitable transparent medium with a refractive index n at a Bragg spacing d. When the Bragg spacing between the reflecting surfaces is an integer number of half waves, the structure becomes optically resonant, with zero electric field intensity at the boundaries and energy coupled through the filter, ideally without loss. Other wavelengths not meeting the resonant condition are reflected. Although most all Bragg gratings are fixed, they are, in theory, tunable by controlling the refractive index n or Bragg spacing d.
The most common Bragg grating is a fiber Bragg grating (FBG) which consists of a fiber segment whose index of refraction varies periodically along its length. Variations of the refractive index constitute discontinuities that emulate a Bragg structure. A periodic variation of the refractive index is formed by exposing the germano-silicate core of the fiber to an intense ultra-violet (UV) optical interference pattern that has a periodicity equal to the periodicity of the grating to be formed. When the fiber is exposed to the intense UV pattern, structural defects are formed and thus a permanent variation of the refractive index having the same periodicity with the UV pattern. The FBG may be tuned by either applying a stretching force that elongates the fiber and thus changes it period (mechanical tuning) or applying heat to elongate the fiber and change its period (thermal tuning).
A similar Bragg grating reflector, based on a stacked-dielectric structure is composed of a quarter-wavelength thick layers, known as a photonic lattice, each with different refractive index. Photonic lattice reflectors have been found to reflect wavelengths over all possible angles of incidence and they do not absorb incident energy, as mirror-based reflectors do.
Bragg gratings are also used in semiconductor waveguides. The grating may be imprinted on the waveguide using a UV approach similar to the FBG or by masking a grating on top of the waveguide. The physical proximity of the grating to the waveguide causes variation of the effective refractive index that constitute discontinuities.
The Bragg grating is similar to the Fabry-Perot filter or etalon, which use a single pair of semireflecting plates. The additional complexity associated with a Bragg grating provides a more well defined passband. The etalon passband is characterized by a very sharp peak and a slow roll-off, which can introduce distortion in the wavelengths that are intended to be transmission and cause crosstalk between neighboring bands. The Bragg grating passband has a flat peak and a very fast roll-off such that the passband wavelengths are transmitted with minimal distortion and crosstalk is reduced.
In view of the above problems, the present invention provides a low cost waveguide tunable Bragg grating that provides a flat passband and minimal crosstalk.
This is accomplished by using a compliant material to form a waveguide that is imprinted with a Bragg grating and mounted on a MEMS actuator. Entropic materials such as elastomers, aerogels or other long-chain polymers may provide the necessary compliance. The application of a drive signal to the actuator deforms (squeezes or stretches) the compliant material thereby changing the Bragg spacing and shifting the resonant wavelength. For example, the MEMS actuator can be an electrostatically or electromagnetically actuated comb-drive.